What are Arachnoid Cysts?

The brain and spinal cord are surrounded by three layers of membranes. These membranes act as a cushioning layer and as physical support for blood supply to and from the brain. The outer membrane is called the dura mater, the middle membrane is called the arachnoid membrane and the inner membrane is called the pia mater.

The arachnoid membrane derives its name from its ‘cobweb-like’ appearance. Arachnoid cysts are fluid-filled sacs located within the arachnoid membrane space. They can be located anywhere along the brain or spinal cord and can be of varying size.

Some arachnoid cysts produce no symptoms or problems and some arachnoid cysts do present problems. Left untreated, arachnoid cysts have the potential to cause permanent neurological damage:

  • Cysts may expand, compressing further on the brain or spinal cord
  • A haemorrhage may injure the brain or spinal cord

Further explanations of arachnoid cysts can be found at the found at the National Institute of Neurological Disorders and Stroke (NINDS) and at "Ask the Doc" in Children First for Health, a clinical site designed for kids (but great for adults, too) sponsored by the Great Ormand Street Hospital for Children, UK. You will find a ‘Useful Definitions’ segment on this website to assist with some of the medical and anatomy jargon.

Malignant or Benign?

All arachnoid cysts are benign. While cysts can expand/grow, they do not spread like a cancer. They are not malignant.


  • ‘Foggy’ head
  • Mental/emotional fatigue
  • Headache
  • Nausea
  • Severe motion sickness/intolerance
  • Squint
  • Seizures
  • Hearing/visual disturbances
  • Vertigo
  • Tinnitus (ringing ears)
  • Numbness in arms and legs